Electronics & Semiconductors — Physics

Q.1

WBCS Prelims 2021

Pure silicon at 0 K is

A. semiconductor

B. metal

C. insulator

D. superconductor

Explanation

Why Correct: At absolute zero (0 K), pure silicon has a completely filled valence band and an empty conduction band separated by a band gap of 1.1 eV, which prevents any electron movement and electrical conduction.
Distractor Analysis: Semiconductors require thermal energy to excite electrons across the band gap, which is unavailable at 0 K. Metals have overlapping valence and conduction bands that allow electron flow even at absolute zero. Superconductors exhibit zero electrical resistance below a critical temperature, but silicon does not display superconductivity at any temperature.
Takeaway: The electrical conductivity of intrinsic semiconductors like silicon increases exponentially with temperature due to thermal excitation of electrons across the band gap.

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Q.2

WBCS Prelims 2020

TV remote control uses

A. Infrared frequency

B. Radio frequency

C. Visible frequency

D. Ultraviolet frequency

Explanation

Why Correct: Standard TV remotes transmit commands via infrared light pulses at wavelengths around 940 nm, just beyond visible red light.
Distractor Analysis: Radio frequencies power wireless communication like Wi-Fi, Bluetooth, and car key fobs. Visible light spans 400-700 nm for human vision. Ultraviolet radiation causes sunburn, sterilizes surfaces, and has wavelengths shorter than 400 nm.
Takeaway: Infrared remotes require line-of-sight, while radio frequency remotes work through obstacles due to longer wavelengths.

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Q.3

WBCS Prelims 2016

With increase in temperature the resistance of a pure semiconductor —

A. first increases and then decreases

B. remains the same

C. increases

D. decreases

Explanation

Why Correct: Pure semiconductors have negative temperature coefficient of resistance — heating creates more charge carriers (electron-hole pairs) by breaking covalent bonds, increasing conductivity and decreasing resistance.
Distractor Analysis: First increases and then decreases describes some doped semiconductors or metals at very low temperatures. Remains the same occurs only in ideal superconductors below critical temperature. Increases characterizes metals and most conductors where lattice vibrations scatter electrons more at higher temperatures.
Takeaway: Metals have positive temperature coefficient (resistance increases with temperature), while semiconductors and insulators have negative temperature coefficient (resistance decreases).

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Q.4

WBCS Prelims 2015

LED stands for

A. Light emitting device

B. Light emitting diode

C. light encapsulated device

D. light emitting dots

Explanation

Why Correct: LED is the standard acronym for Light Emitting Diode, a semiconductor device that emits light when current flows through it in the forward direction.
Distractor Analysis: Light emitting device is a generic term for any light-producing device. Light encapsulated device refers to packaging technology but not the semiconductor component itself. Light emitting dots describes quantum dot technology, which differs from traditional semiconductor LEDs.
Takeaway: LEDs operate on electroluminescence principle and are more energy-efficient than incandescent bulbs, converting about 90% of energy to light compared to 10% for traditional bulbs.

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Q.5

WBCS Prelims 2014

LCD stands for

A. Liquid Crystal Display

B. Low Current Display

C. Light Circuit Display

D. None of the above

Explanation

Why Correct: LCD expands to Liquid Crystal Display, a flat-panel technology using liquid crystals to modulate light for images.
Distractor Analysis: Low Current Display describes no established display technology. Light Circuit Display is not a recognized technical term. None of the above fails because Liquid Crystal Display is the correct and standard expansion.
Takeaway: LED stands for Light Emitting Diode, a display technology using semiconductor diodes to emit light directly.

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Q.6

WBCS Prelims 2007

Materials like Germanium and Silicon are called

A. Insulators

B. Conductors

C. Metals

D. Semi-conductors

Explanation

Why Correct: Germanium and Silicon have electrical conductivity between conductors and insulators, typically with resistivity around 10^-3 to 10^6 ohm-m, and their conductivity can be precisely controlled through doping for electronic applications.
Distractor Analysis: Insulators like rubber or glass have very high resistivity (>10^8 ohm-m) and negligible conductivity. Conductors like copper or silver have low resistivity (<10^-5 ohm-m) with high conductivity. Metals are a specific class of conductors characterized by metallic bonding and free electron movement.
Takeaway: Pure semiconductors have four valence electrons forming covalent bonds, with conductivity increasing with temperature (unlike metals) due to thermal excitation across their 0.7-1.1 eV band gap.

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Q.7

WBCS Prelims 2004

In a p-type semiconductor, which is the main carrier of electricity?

A. Positive holes

B. positive ions

C. Electrons

D. Negative ions

Explanation

Why Correct: P-type semiconductors are doped with trivalent impurities like boron that create electron vacancies called holes, which act as mobile positive charge carriers.
Distractor Analysis: Positive ions are stationary dopant atoms with missing electrons that remain fixed in the crystal lattice. Electrons are the minority carriers in p-type semiconductors and the majority carriers in n-type semiconductors. Negative ions are stationary dopant atoms with extra electrons that do not conduct current.
Takeaway: In n-type semiconductors, pentavalent impurities like phosphorus donate free electrons as majority carriers.

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